The present invention relates to a lithographic printing plate precursor, and specifically a lithographic printing plate precursor comprising a support having provided thereon a photosensitive layer (also referred to as an image-forming layer) capable of plate-making by scanning exposure based on digital signals, and capable of water development, or capable of mounting on a printing machine for performing printing with requiring no development.
A lithographic printing plate generally comprises a lipophilic image area which receives an ink and a hydrophilic non-image area which receives fountain solution during printing. As such a lithographic printing plate precursor, a PS plate (presensitized plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer has so far been widely used, and the plate-making method generally comprises performing mask exposure through a lith film and then dissolving and removing a non-image area with a developing solution to thereby obtain a printing plate.
Digitized techniques of electronically processing image data with a computer have prevailed in recent years, and various image output systems corresponding to these digitized techniques have been put to practical use. With such a trend, a computer-to-plate technique of directly making a printing plate by scanning digitized image data with highly directive actinic radiation such as laser beams without using a lith film has been earnestly desired, and it has become an important technical problem to obtain a printing plate precursor well adapted to this purpose.
On the other hand, the plate-making process of the PS plate hitherto in use is indispensably accompanied by wet process of dissolution and removal of a non-image area after exposure, this is another problem which has been desired to be improved. In particular, global environmental protection has been a matter of concern in the industry at large in recent years. There are hence increased demands for simplification of processing, switching over to dry process, and no processing from the viewpoint of environmental aspect and rationalization of the process with digitization.
As one plate-making method which does away with former processing steps, there is a development on machine system of using a photosensitive layer capable of removing the non-image area of a printing plate precursor in usual printing process without carrying out former development process, and effecting development after exposure on a printing machine to thereby obtain a final printing plate. However, one big problem of the development on machine system is that the printing plate precursor must be stored under a completely light-shielded state and/or under a constant temperature condition after exposure, e.g., during the period of time until the printing plate precursor is mounted on a printing machine, because the fixation of the photosensitive layer is not performed.
On the other hand, solid state lasers having high output, e.g., a semiconductor laser and a YAG laser are inexpensively available in recent years. As a result, a method of using these lasers is promising as a plate-making method by scanning exposure. In the method of high power density exposure using these high output lasers, various development systems can be utilized differing from photo-reactions used in photosensitive materials for low to middle power density exposure. Light energy absorbed by photosensitive materials is converted to heat and desired development is caused by the generated heat.
A big advantage of a plate-making method utilizing heat mode recording is that the material is not sensitized by exposure to light of general illuminance level and in normal atmospheric temperature, and fixation of the image after exposure is not essential. Accordingly, for example, when a photosensitive layer which is insolubilized or solubilized by heat mode exposure is used in a plate-making process by the on-press development system, it becomes possible to realize a system in which the image obtained is not influenced even development (removal of a non-image area) is performed after the printing plate precursor is exposed to atmospheric light for a certain period of time after image exposure.
Accordingly, if heat mode recording is utilized, it will be possible to obtain a lithographic printing plate precursor which is adapted to the on-press development system.
A method is suggested as one preferred plate-making method of a lithographic printing plate based on heat mode recording, which comprises the steps of providing a hydrophobic image-forming layer on a hydrophilic support, imagewise exposing the image-forming layer by heat mode exposure to convert the solubility/dispersibility of the hydrophobic image-forming layer, and removing a non-image area by wet development, according to necessity.
However, the image-forming layer as above is not sufficient in heat sensitivity, hence the sensitivity to heat mode scanning exposure is extremely unsatisfactory. Further, it is also a problem in practical use that the discrimination of hydrophobicity/hydrophilicity before and after exposure, i.e., the change in solubility, is small. It is almost impossible to perform plate-making by the on-press development system with poor discrimination.
Accordingly, an object of the present invention is to provide a lithographic printing plate precursor which is capable of plate-making by scanning exposure with a solid state laser and a semiconductor laser emitting infrared rays based on digital signals, which is high sensitivity, and causes no stains due to residual films.
Another object of the present invention is to provide a lithographic printing plate precursor which can be developed by water or an aqueous solution, or can be mounted on a printing machine to perform printing with requiring no development.
As a result of eager investigation of the present inventors for achieving the above objects, it has been found that the above problems have been solved by the following lithographic printing plate precursor, thus the present invention has been accomplished.
That is, the present invention is as follows.
(1) A lithographic printing plate precursor comprising a support having a hydrophilic surface having provided thereon an image-forming layer containing a hydrophobic high molecular compound having at least either a functional group represented by formula (1) or a functional group represented by formula (2): 
wherein X+ represents an iodonium ion, a sulfonium ion or a diazonium ion.
(2) A lithographic printing plate precursor comprising a support having a hydrophilic surface having provided thereon an image-forming layer containing a hydrophobic infrared ray absorber having at least either a functional group represented by formula (1) or a functional group represented by formula (2): 
wherein X+ represents an iodonium ion, a sulfonium ion or a diazonium ion.
(3) The lithographic printing plate precursor as described in the above item (1) or (2), wherein the image-forming layer contains a compound having at least either a functional group represented by formula (3) or a functional group represented by formula (4): 
wherein R1 and R2 each represents a hydrogen atom, an alkyl group, an aryl group, an alkynyl group or an alkenyl group; R3 represents an alkyl group, an aryl group, an alkynyl group or an alkenyl group; R4 represents a hydrogen atom, an alkyl group, an aryl group, an alkynyl group or an alkenyl group; either R5 or R6represents a hydrogen atom and the other represents a hydrogen atom, an alkyl group, an aryl group, an alkynyl group or an alkenyl group; and arbitrary two of R1, R2 and R3 may form a ring, and arbitrary two of R4, R5 and R6 may form a ring.
The iodonium ion, sulfonium ion and diazonium ion represented by X+ are well known in the industry as acid-generating agents and they form the acids of corresponding counter anions by irradiation with actinic rays and/or heating. In conventional lithographic printing plates, the thus-generated acids have been used in a crosslinking reaction or as the catalysts to cause the decomposition of acid-decomposable functional groups.
Contrary to this, according to the lithographic printing plate precursor of the present invention, a sulfonate group and a carboxylate group such as the above functional groups are converted to a sulfonic acid and a carboxylic acid respectively by irradiation with actinic rays or heating, and the originally hydrophobic high molecular compound changes to hydrophilic, by which the image-forming layer is also converted to hydrophilic. By developing the lithographic printing plate precursor with water, an aqueous liquid or a fountain solution on a printing machine, the image-forming layer of the heated area is dissolved and removed, thereby a lithographic printing plate is made.
Further, the infrared ray absorber contained in the image-forming layer has at least either a functional group represented by formula (1) or a functional group represented by formula (2), and the infrared ray absorber changes to hydrophilic by irradiation with actinic rays and/or heating due to having the functional group. Therefore, the infrared ray absorber does not remain as a residual color at the exposed area, or does not form scummy solid phase in a fountain solution during printing, thus an excellent lithographic printing plate which does not cause stains can be obtained.
Further, a compound having functional groups represented by formula (3) and/or (4) is contained in the image-forming layer of the lithographic printing plate precursor. By containing the compound, it is possible to change the image-forming layer to soluble in an aqueous liquid with less energy. The cause of this fact is not clear but it is thought due to the following mechanism. 
As described above, the lithographic printing plate precursor according to the present invention is capable of direct plate-making from digital data of a computer by recording with a thermal head, a solid state laser emitting infrared rays and a semiconductor laser, or a solid state laser emitting visible rays and a semiconductor laser, and a lithographic printing plate showing high sensitivity and high press life and not causing stains can be obtained.